1. Field of the Invention
Embodiments of the present invention relate to a display device and, more particularly, to a liquid crystal display (LCD) device and a fabrication method thereof. Although embodiments of invention are suitable for a wide scope of applications, it is particularly suitable for simplifying a fabrication process and improving a production yield by reducing the number of masks by using a two-metal stacked structure and also suitable for forming a forward taper shape in the two-metal stacked structure.
2. Background of the Related Art
As the consumer's interest in information displays is growing and the demand for portable (mobile) information devices is increasing, research and commercialization of light and thin flat panel displays (“FPD”) has increased.
Among FPDs, the liquid crystal display (“LCD”) is a device for displaying images by using optical anisotropy of liquid crystal. LCD devices exhibit excellent resolution and color and picture quality, so it is widely used for notebook computers or desktop monitors, and the like.
The LCD includes a color filter substrate, an array substrate and a liquid crystal layer formed between the color filter substrate and the array substrate.
An active matrix (AM) driving method commonly used for the LCD is a method in which liquid crystal molecules in a pixel part are driven by using amorphous silicon thin film transistors (a-Si TFTs) as switching elements.
In the fabricating process of the LCD, a plurality of masking processes (namely, photographing processes) are basically performed to fabricate the array substrate including the TFTs, so a method for reducing the number of masking process will increase productivity.
The general structure of the LCD will now be described in detail with reference to FIG. 1.
FIG. 1 is an exploded perspective view showing a general LCD.
As shown in FIG. 1, the LCD includes a color filter substrate 5, an array substrate 10 and a liquid crystal layer 30 formed between the color filter substrate 5 and the array substrate 10.
The color filter substrate 5 includes a color filter (C) including a plurality of sub-color filters 7 that implement red, green and blue colors, a black matrix 6 for dividing the sub-color filters 7 and blocking light transmission through the liquid crystal layer 30, and a transparent common electrode 8 for applying voltage to the liquid crystal layer 30.
The array substrate 10 includes gate lines 16 and data lines 17 which are arranged vertically and horizontally to define a plurality of pixel regions (P), TFTs, switching elements, formed at respective crossings of the gate lines 16 and the data lines 17, and pixel electrodes 18 formed on the pixel regions (P).
The color filter substrate 5 and the array substrate 10 are attached in a facing manner by a sealant (not shown) formed at an edge of an image display region to form a liquid crystal panel, and the attachment of the color filter substrates 5 and the array substrate 10 is made by an attachment key formed on the color filter substrate 5 or the array substrate 10.
FIGS. 2A to 2E are cross-sectional views sequentially showing a fabrication process of the array substrate of the LCD in FIG. 1.
As shown in FIG. 2A, a gate electrode 21 made of a conductive material is formed by using a photolithography process (a first masking process) on a substrate.
Next, as shown in 2B, a first insulation film 15a, an amorphous silicon thin film and an n+ amorphous silicon thin film are sequentially deposited over the entire surface of the substrate 10 with the gate electrode 21 formed thereon, and the amorphous silicon thin film and the n+ amorphous silicon thin film are selectively patterned by using the photolithography process (a second masking process) to form an active pattern 24 formed of the amorphous silicon thin film on the gate electrode 21.
In this case, the n+ amorphous silicon thin film pattern 25 which has been patterned in the same form as the active pattern 24 is formed on the active pattern 24.
Thereafter, as shown in FIG. 2C, a conductive metal material is deposited over the entire surface of the array substrate 10 and then selectively patterned by using the photolithography process (a third masking process) to form a source electrode 22 and a drain electrode 23 at an upper portion of the active pattern 24. At this time, a certain portion of the n+ amorphous silicon thin film pattern formed on the active pattern 24 is removed through the third masking process to form an ohmic-contact layer 25′ between the active pattern 24 and the source and drain electrodes 22 and 23.
Subsequently, as shown in FIG. 2D, a second insulation film 15b is deposited over the entire surface of the array substrate 10 with the source electrode 22 and the drain electrode 23 formed thereon, and a portion of the second insulation film 15b is removed through the photolithography process (a fourth masking process) to form a contact hole 40 exposing a portion of the drain electrode 23.
As shown in FIG. 2E, a transparent conductive metal material is deposited over the entire surface of the array substrate 10 and then selectively patterned by using the photolithography process (a fifth making process) to form a pixel electrode 18 electrically connected with the drain electrode 23 via the contact hole 40.
As mentioned above, in fabricating the array substrate including TFTs, according to the related art, a total of five photolithography processes are necessarily performed to pattern the gate electrode, the active pattern, the source and drain electrodes, the contact hole and the pixel electrode.
A photolithography process is a process of transferring a pattern formed on a mask onto the substrate on which a thin film is deposited to form a desired pattern, which includes a plurality of processes such as a process of coating a photosensitive solution, an exposing process and a developing process, etc, which degrade the production yield.
In particular, because the masks designed for forming the pattern are quite expensive, as the number of masks used in the processes increases, the fabrication cost of the LCD increases proportionally.
A technique for fabricating the array substrate by performing the masking process four times by forming the active pattern and the source and drain electrodes using a single masking process having a slit (diffraction) mask has been proposed.
However, because the active pattern, the source and drain electrodes and the data lines are simultaneously patterned by performing an etching process twice, the active pattern protrusively remains near the lower portions of the source electrode, the drain electrode and the data lines.
The protrusively remaining active pattern is formed of an intrinsic amorphous silicon thin film, so the protrusively remaining active pattern is exposed to light from the lower backlight, generating an optical current. The amorphous silicon thin film reacts slightly to a blinking of the light from the back light, and repeatedly becomes activated and deactivated, which causes a change in the optical current. The changing optical current component is coupled with a signal flowing in the neighboring pixel electrodes so as to distort movement of the liquid crystal molecules positioned at the pixel electrodes. As a result, a wavy noise is generated such that a wavy fine line appears on a screen of the LCD.
In addition, because the active pattern positioned at the lower portion of the data lines has portions that protrude at a certain height from both sides of the data lines, the opening region of the pixel part is encroached by as much as the protrusion height, thus resulting in a reduction in an aperture ratio of the LCD.